I've seen many people close their gaps by just wrapping small rubber bands around their teeth. is that safe to do?

It is safe : However efficiency is doubtful

NO, not safe: The problem with this idea is that the rubber bands often slip down the tooth and then under the gum. This force can lead to bone loss and periodontal disease and if left unchecked can actually cause extraction of the tooth. Use of rubber bands for this purpose is both dangerous and foolhardy.

Probably not: this is a dangerous way to close space. probably supervised closure is better

Very dangerous: First that's only a temporary solution, gap will eventually reopen. Second, if you close gap in middle you open 2 smaller gaps on either side. Third, if rubber band slips under gum line it will cause loss of attachment and teeth will fall out. You need professional care to do this properly. Your friends should see their Dentists immediately to check integrity of the Gingival Attachment.

Not safe: Uncontrolled movement and forces can do more harm than good. And if you manage to move the teeth, how will you keep them in place? See an orthodontist for proper care.